Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products and especially, but not exclusively, portable products such as mobile phones, audio players, video players, personal digital assistants (PDAs), various wearable devices, mobile computing platforms such as laptop computers or tablets and/or games devices. Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers.
Micro-electromechanical-system (MEMS) transducers, such as MEMS microphones are finding application in many of these devices. There is therefore also a continual drive to reduce the size and cost of the MEMS devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive that are deposited on or within the membranes and/or a substrate or back-plate. In the case of MEMS pressure sensors and microphones, the electrical output signal is usually obtained by measuring a signal related to the capacitance between the electrodes. However in some cases the output signal may be derived by monitoring piezo-resistive or piezo-electric elements. In the case of capacitive output transducers, the membrane is moved by electrostatic forces generated by varying a potential difference applied across the electrodes, though in some other output transducers piezo-electric elements may be manufactured using MEMS techniques and stimulated to cause motion in flexible members.
To provide protection, the MEMS transducer may be contained within a package. The package effectively encloses the MEMS transducer and can provide environmental protection while permitting the physical input signal to access the transducer and providing external connections for the electrical output signal.
FIG. 1a illustrates one conventional MEMS microphone package 100a. A MEMS transducer 101 is attached to a first surface of a package substrate 102. The MEMS transducer 101 may typically be formed on a semiconductor die by known MEMS fabrication techniques. The package substrate 102 may be silicon or a printed circuit board (PCB) or a ceramic laminate or any other suitable material. A cover 103 is located over the transducer 101 attached to the first surface of the package substrate 102. The cover 103 may be a metallic lid. An aperture, i.e. hole, 104 in the cover 103 provides a sound port and allows acoustic signals to enter the package. In this example, the transducer 101 is wire bonded from bond pads 105 on the transducer to bond pads 105a on the package substrate 102. Electrical pathways in or on the substrate connect between the bond pads on the internal face of the substrate and lead, i.e. solder, pads 108 on the external face of the substrate to provide an external electrical connection to the transducer.
The sound port, or acoustic port, 104 allows transmission of sound waves to/from the transducer within the package. The transducer may be configured so that the flexible membrane is located between first and second volumes, i.e. spaces/cavities that may be filled with air (or some other gas suitable for transmission of acoustic waves), and which are sized sufficiently so that the transducer provides the desired acoustic response. The sound port 104 acoustically couples to a first volume on one side of the transducer membrane, which may sometimes be referred to as a front volume. The second volume, sometimes referred to as a back volume, on the other side of the one or more membranes, is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed (although it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths such as bleed holes, i.e. small holes in the membrane, that are configured so as to present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials due to temperature changes or the like).
FIG. 1b illustrates another known MEMS transducer package 100b. Again, a transducer 101, which may be a MEMS microphone, is attached to the first surface of a package substrate 102. In this example, the package 100b also contains an integrated circuit 106, which although not illustrated may also be present in FIG. 1a. The integrated circuit 106 may be provided for operation of the transducer and may, for example, be a low-noise amplifier for amplifying the signal from a MEMS microphone. The integrated circuit 106 is electrically connected to electrodes of the transducer 101 and is also attached to the first surface of the package substrate 102. The integrated circuit 106 is electrically connected to the transducer 101 via wire-bonding. A cover 107 is located on the package substrate so as to enclose the transducer 101 and the integrated circuit 106. In this package, the cover 107 is a two-piece cover that comprises an upper part or lid portion 107a and a spacer or frame portion 107b surrounding a cavity in which the transducer 101 and the integrated circuit 106 are situated. The package substrate 102, cover and frame portion may all be formed of PCB or ceramic material which may be multi-layer laminate structures. The cover 107 has a sound port 104 in the upper part 107a which allows acoustic signals to enter the package. Each of the substrates in FIGS. 1a and 1b have external lead pads, i.e. solder pads, 108 for external connection to an end user's PCB via a solder reflow process for example.
In order to buffer the generally weak transducer output signal, an integrated circuit amplifier circuit may also be used in the packages similar to that shown in FIG. 1a and connected internally in similar fashion to that shown in FIG. 1b. In some examples, the acoustic port may be provided through the substrate 102 rather than the cover, or sometimes in both to provide a differential or directional microphone.
Various other styles of packages for MEMS microphone and other MEMS transducers are available, but may be complex multi-part assemblies and/or require physical clearance around the transducer for connections, impacting material and manufacturing cost and physical size.
The embodiments disclosed herein relate to improved MEMS transducer packages.